In recent years, modern communications technology has provided various systems for position determination. The global positioning system (GPS) operated by the United States Department of Defense, for example, is a highly complex system of determining the position of an object. The GPS system depends on measuring the time-of-flight of microwave signals from three or more orbiting satellite transmitters by a navigation receiver that computes the position of the mobile unit. According to the GPS system, each satellite broadcasts a time-stamped signal that includes the satellite's ephemeris, i.e., its own position. When the mobile unit receives a GPS signal, the mobile unit measures the transmission delay relative to its own clock and determines the pseudo-range to the transmitting satellite's position. The GPS system requires three satellites for two-dimensional positioning, and a fourth satellite for three-dimensional positioning.
Another approach is that employed by the U.S. Navy's TRANSIT system. In that system, a mobile unit performs continuous doppler measurements of a signal broadcast by a low earth orbit (LEO) satellite. The measurements continue for several minutes. The system usually requires two passes of the satellite, necessitating a wait of more than 100 minutes. In addition, because the position calculations are performed by the mobile unit, the satellite must broadcast information regarding its position, i.e., its ephemeris. Although the TRANSIT system is capable of high accuracy (on the order of one meter), the delay required is unacceptable for commercial applications.
Although these systems accurately determine the unknown position of an object, they are extremely complex, and, more importantly, expensive to implement. For example, both the GPS and TRANSIT systems require multiple satellites, sophisticated receivers and antennas that require hundreds of millions dollars of investments. Also, response times of GPS and TRANSIT systems are typically slow due to their narrow bandwidth. Furthermore, since these systems depend on orbiting satellites, they require an unimpeded view of the sky to operate effectively.
There is a great need in many different fields for a simple, less expensive alternative to complicated position determination systems. One such area is a typical shipping terminal, e.g., a major sea-port or an airport. In a sea-port, containers having valuable cargo are stored at warehouses or are left in designated places in the terminals. Also, containers are sometimes moved from one section of the port to another section in preparation for their eventual loading into a cargo ship or being picked up by trucks or railcars after being unloaded from a cargo ship. Often it is necessary to determine the location of one or more containers. However, it is difficult to identify one or more containers among hundreds, or thousands of containers in a terminal. Similar problems are also encountered in airports and railway terminals where containers are kept in storage sites.
A simple, less expensive position determination system is also desirable for locating police units. Such a position determination system can be used as a vehicle locator system. A city dispatcher would be able to quickly and efficiently dispatch police units if the dispatcher has pre-existing knowledge of each unit's locations. Currently city dispatchers use mobile phones to communicate with police units in order to know their locations. However, using mobile phones to determine the positions of the police units has some disadvantages. Use of mobile phones is expensive and time consuming. Also, when a police officer is not in the car, it is not possible to determine the unit's location.
Recently, the FCC has mandated that all cell phone systems implement position determination for use in emergency call location. In addition, there is a need for position determination as part of cell phone security, fraudulent use, and zone handoff algorithms. These requirements are difficult to meet and GPS is not adequate to reliably deliver the required accuracy.
Systems comprised of two or more radio transceivers have been developed which are intended to locate and track objects within the scope or zone of the system, including other transceivers. Necessary to this function is the capability of the transceivers to accurately measure distance, whether it is the distance to the object being tracked or distance to another transceiver. However, the distances measured typically only take into account the distance from the antenna of one radio to the antenna of the other radio while the actual distance of the transmitter of the signal from the antenna may be greater, thus making distance determination inaccurate. Even distances of signal travel within the transceiver circuitry can be as much as forty feet. Compounding this error is the fact that as the circuitry warms, signal time of flight through circuitry may increase due to reaction of the circuitry to heat.
Known in the art is the technique of time domain reflectometry (TDR) which typically measures the length of a wire by sending a pulse down the wire and timing the arrival of the reflection. In this manner, one is able to pinpoint breaks in underground cable, for example. However, to applicant's knowledge this technique has not been used to determine distance of the internal circuitry in a transceiver apparatus because reception of energy while the transceiver is transmitting has been blocked by use of a transmit receive switch. Typically, transmit receive switches protect receive circuitry from the power associated with signal transmission in order to prevent “blinding” the receiver to incoming signals. This has prevented use of TDR to measure internal transceiver circuitry.
Thus, a system and method is needed to enhance the accuracy of positioning systems and to provide feedback and re-calibration in case of measurement drift due to temperature variations.